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Concept
S Doradus
Summary
S Doradus (also known as S Dor) is one of the brightest stars in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, located roughly 160,000 light-years away. The star is a luminous blue variable, and one of the most luminous stars known, having a luminosity varying widely above and below 1,000,000 times the luminosity of the Sun, although it is too far away to be seen with the naked eye.
S Doradus was noted in 1897 as an unusual and variable star, of Secchi type I with bright lines of Hα, Hβ, and Hγ. The formal recognition as a variable star came the assignment of the name S Doradus in 1904 in the second supplement to Catalogue of Variable Stars.
S Dor was observed many times over the coming decades. In 1924, it was described as "P Cygni class" and recorded at photographic magnitude 9.5 In 1925, its absolute magnitude was estimated at −8.9. In 1933 it was listed as a 9th-magnitude Beq star with bright hydrogen lines. It was the most luminous star known at that time.
In 1943, the variability was interpreted as being due to eclipses of a binary companion, orbiting with a period of 40 years. This was refuted in 1956, when the variability was described as irregular and the spectrum as A0 with s and emission for many spectral lines. The brightness was observed to decline by 0.8 magnitude from 1954 into 1955. At the same time, S Doradus was noted as being similar to the Hubble–Sandage variables, the LBVs discovered in M31 and M33. The brief 1955 minimum was followed by a deep minimum in 1964, when the spectrum was compared to Eta Carinae in strong contrast to the mid-A spectrum at normal brightness.
By 1969 the nature of S Doradus was still uncertain, considered possibly to be a pre-main-sequence star, but during the next decade the consensus settled on the S Doradus type variables and Hubble-Sandage variables being evolved massive supergiants. They were eventually given the name "luminous blue variables" in 1984, coined in part because of the similarity of the acronym LBV to the well-defined LPV class of variable stars.
Official source
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